For purifying exhaust gas containing, for example, CO, HC, NO, and NO2, precious metals (Pt, Rh, Pd and Ir) exhibit high performance. Therefore, it is preferable to employ the above-mentioned precious metals in the purification catalyst for exhaust gas. These precious metals are generally mixed with or supported by Al2O3 of high surface-to-weight ratio together with additives such as La, Ce, and Nd. On the other hand, composite oxides (for example, a perovskite-like oxide), made by combining various elements, have extremely varied properties. Therefore, it is preferable for a purification catalyst for exhaust gas to employ the above-mentioned composite oxides. Moreover, when the precious metal is supported by the composite oxides, the properties of precious metal are significantly changed. From this viewpoint, a preferable performance for purifying exhaust gas can be obtained in the purification catalyst for exhaust gas in which a precious metal is supported by a composite oxide.
Various catalysts mentioned above are now being developed, and for example, a technique in which a coalescence rate of the precious metal can be reduced by having a perovskite be a support, judging from deterioration of the precious metal with reduction of active sites by coagulation of the precious metal, is proposed (see claims of Japanese Unexamined Application Publication No. 5-86259). Moreover, another technique in which reduction of PdO can be reduced by using a perovskite in which the A site is defective, judging from reducing PdO which is an activated species in a NO reduction reaction, whereby the PdO changes to Pd which is low-active Pd, when the precious metal is Pd, is proposed (see the claims of reactions disclosed in Japanese Unexamined Application Publication No. 2003-175337). Usually, precious metals are used on a support of Al2O3 or the like, either alone or in combination, but in severe conditions such as in an automobile, active sites decrease due to coagulation, and the activity drops substantially. To solve this problem, it is proposed to use precious metals together with other elements in a form of composite oxides. As for Pd, in particular, composite oxides of rare-earth metals and Pd have been disclosed (see the claims of Japanese Unexamined Application Publication No. S61-209045, the claims of Japanese Unexamined Application Publication No. H1-43347, the claims of Japanese Unexamined Application Publication No. H4-27433, the claims of Japanese Unexamined Application Publication No. H4-341343, the claims of Japanese Unexamined Application Publication No. H7-88372, and the claims of Japanese Unexamined Application Publication No. H10-277393).
Conventional purification catalysts for exhaust gas exhibit sufficient performance for reducing CO, HC, and NOx (NO, NO2, etc.) contained in exhaust gas, in a running of vehicle, particularly during running at high temperatures (not less than 400° C.). However, the conventional catalysts cannot exhibit sufficient performance for reducing CO, HC, and NOx, in a vehicle at the starting or idling thereof at low temperatures (below 400° C.).
As mentioned above, the reason that sufficient performance for purifying the exhaust gas cannot be obtained in the running at low temperature is as follows. That is, in the conventional purification catalyst for exhaust gas, a precious metal, for example, Pt, Rh, or Pd, is supported on Al2O3 having a high surface-to-weight ratio. Due to the high surface-to-weight ratio of the Al2O3, the precious metal is advantageously supported in a highly dispersed condition. However, Al2O3 is a stable compound, and does not mutually affect a supported precious metal, whereby activity of the precious metal is not improved. Accordingly, sufficient performance during the running at low temperature may not be obtained.
Moreover, in the running of a vehicle, it is preferable for Pd to exist in a condition of PdO which is highly reactive. However, even if Pd supported on the Al2O3 initially exists in a condition of PdO, the Pd is reduced to be in a metal condition at high temperatures (not less than 900° C.), and as Pd coagulates, active sites decrease, whereby the activity is significantly reduced.